1. Field of the Invention
The present invention relates to a control apparatus for a vehicle incorporating an automatic transmission including a hydraulic coupling with a lock-up clutch, and particularly to a technique of learning, in controlling engagement force of the lock-up clutch so that a slip amount thereof meets a predetermined slip amount, a control value for obtaining the predetermined slip amount.
2. Description of the Background Art
An automatic transmission for a vehicle is constituted of a hydraulic coupling connected to an output shaft of an engine and a transmission mechanism of gear type or a continuously variable transmission mechanism of belt type or traction type connected to the output shaft of the hydraulic coupling. An example of the hydraulic coupling is a torque converter, which often has a lock-up clutch that enables direct coupling of the input side and the output side of the torque converter. Engagement of the lock-up clutch provides control of directly coupling the input side and the output side of the torque converter, and provides feedback control (slip control) for attaining a prescribed state of the engagement pressure of the lock-up clutch in accordance with the difference between the number of revolutions of the pump on the input side (corresponding to the number of revolutions of the engine) and the number of revolutions of the turbine on the output side to thereby provide control of the slip state of the torque converter.
As such slip control, for example Japanese Patent Laying-Open No. 07-035232 discloses a slip control apparatus for a lock-up clutch for a vehicle, wherein appropriate slip control is provided even when an engine output fluctuates due to a change in the outside air pressure or the like. The slip control apparatus includes slip controlling means for determining a control input for the slip control in accordance with values of control parameters from a data map that is preset so that a target slip amount is obtained based on control parameters prescribed with reference to a predetermined standard engine output state, and for controlling the engagement force of the lock-up clutch in accordance with the control input: The slip control apparatus includes learning means for updating the control input of the data map in accordance with the control parameters so that the slip amount of the lock-up clutch meets the target slip amount, output fluctuation detecting means for detecting whether or not an engine output state fluctuates from the standard engine output state, and learning limiting means for prohibiting update of the data map by the learning means when the fluctuation of the engine output state is detected by the output fluctuation detecting means.
According to the slip control apparatus, the control input of the data map is learned corresponding to the control parameters so that the slip amount of the lock-up clutch meets the target slip amount, and the engagement force of the lock-up clutch is controlled in accordance with the learned control input. Accordingly, appropriate slip control can always be provided irrespective of individual differences among engines or changes in the engine output over time. Additionally, since the learning of the control input is prohibited upon detection of the fluctuation in the engine output state from the standard engine output state, erroneous slip control will not be provided when the engine output state returns to the standard engine output state.
However, when an engagement range of the lock-up clutch is increased in order to improve the fuel efficiency, a slip control range is set smaller and the frequency of slip control may be reduced. Hence, there is a problem that the frequency of learning the control value of the engagement force of the lock-up clutch during the slip control is reduced. When the frequency of learning is reduced, the accuracy of the slip control is deteriorated. Thus, hunting where the lock-up clutch repeats engagement and disengagement in the slip control range may occur. Further, the vehicle may be shocked by the hunting.